In electrochemical devices of all kinds, and, in particular, those battery systems having a plurality of cells immersed in a common electrolyte, shunt current losses are known to result from conductive current bypass paths which occur in the electrolyte surrounding the cells. Such shunt current losses are always present in these devices during charging, discharging and under open circuit conditions, and have undesirable side effects leading to the shortening of their useful life.
In a patent application assigned to a common assignee, by M. Zahn, P. G. Grimes, and R. J. Bellows, entitled, "SHUNT CURRENT ELIMINATION AND DEVICE", Ser. No. 939,325, filed Sept. 5, 1978, now U.S. Pat. No. 4,197,167, a method for eliminating shunt currents is described wherein a protective nulling current is applied through the common electrolyte disposed in a common manifold. The present invention is based upon the teachings set forth in the above-mentioned application, and is meant to incorporate these teachings herein by way of reference.
In the course of applying these protective currents, particularly in systems featuring circulating electrolyte, electrodes are required which would not block or impede the flow of electrolyte through the manifold system. One of the simplest and most effective electrode designs from a hydraulic point of view is a thin wire electrode disposed in the center of the flow stream. These electrodes prevent substantial pressure drops. This type of electrode applies the current at a point of focused source, and, therefore, provides a non-uniform current density profile in the manifold in the proximity of the electrode, which current density eventually spreads along the length of the manifold. Devices which show a point source or focused current electrode structure are to be seen in the patents to P. Durand, entitled "ELECTROCHEMICAL GENERATORS WITH AUXILIARY CATHODE", U.S. Pat. No. 4,136,232, issued Jan. 23, 1979, and J. Jacquelin, entitled "FORCED FLOW ELECTROCHEMICAL BATTERY", U.S. Pat. No. 4,081,585, issued Mar. 28, 1978.
The Jacquelin and Durand electrodes are constructed in this fashion in order to generate finite amounts of zinc metal, which are easily washed away in the electrolyte stream. The non-uniformity of the current density with the use of a point source electrode along the manifold will produce a voltage drop at each cell position which does not equal the shunt voltage. The shunt currents would, therefore, not be effectively reduced using these electrodes.
The invention first contemplated the use of a screen or mesh type electrode for allowing a generally unimpeded fluid flow, while also providing a substantially uniform current density to be applied throughout the manifold. While such an electrode structure would work well in some cases, it was impractical where the electrode would be required to supply reactants to the electrolyte solution, or remove certain undesirable products therefrom. For example, in an electrolysis cell, the need to remove oxygen build-up in the hydrogen production electrolyte is desirable to prevent the possibility of an explosion. Also, in certain situations, the pressure drop in the fluid caused by screens is undesirable.
After careful assessment of all the feasible electrode designs, the use of an annular-type of electrode was deemed to be the most practical. The annular electrode does not impede or block fluid flow in those systems or parts of systems using circulating electrolyte, and at the same time will allow for the application of a protective current about the manifold which provides a substantially uniform current density profile through the electrolyte along the manifold.
In addition, the annular electrode can be provided with means to inject or remove its reactants and/or its products from the electrolyte.
As a further advantage of applying a protective current about the manifold, which current has a uniform current density profile, there is a reduction of the power required to maintain the nulling current.
The reduction in power consumption with the use of an annular electrode structure is made possible by the fact that other electrode designs require that the electrodes be placed at a greater distance along the manifold from the individual current producing cells. This greater distance is necessary to allow the lines of current from the point source to radially spread-out into the manifold, so as to provide a proper nulling match with the shunt voltage at each cell position. Because the currents have to travel through a greater electrolyte distance with the use of a point source electrode, a greater voltage boost or power consumption is required for these point or focused electrode designs. Annular electrodes which put forth a substantially uniform pattern of current lines in the manifold can be placed closer to the cells, thus requiring less power.
In addition, since impressed electrode voltage is also a function of current density, the design of electrodes with a greater surface area, as available with annular electrode designs, is also desirable from a current density and power standpoint.